Three U.S. senators have sent a letter to NCAA president Mark Emmert expressing concern about the NCAA's oversight of its member schools and asking him to respond to a series of requests, including one for copies of all the association's broadcast rights contracts.

The letter, dated May 5, was released by Sens. Claire McCaskill (D-Mo.), Jay Rockefeller (D-W. Va.) and Cory Booker (D-N.J.) a day after the Senate Committee on Commerce, Science and Transportation announced the postponement of a hearing concerning college sports that it had scheduled for Wednesday. The hearing was titled "Promoting the Well-Being and Academic Success of College Athletes." A news release from the senators said the hearing will be held "in the coming weeks."

Rockefeller chairs the committee, of which McCaskill and Booker are members.

The release by the senators said: "The NCAA is a tax exempt, non-profit organization founded specifically to protect and promote the well-being of student-athletes. However, reports of exploitation at member institution have become commonplace."

The letter references a response that Emmert had provided in November 2013 to a request for information from Rockefeller and McCaskill, then says that the response "leaves us with the impression that the National Collegiate Athletic Association ... defers to member institutions on most matters potentially leaving student-athletes vulnerable to the very abuses the NCAA was created to protect against.

"As colleges and universities generate growing revenue and publicity with each passing year for colleges and universities, the NCAA, and sponsors, the potential for exploitation and abuse of student-athletes has never been greater. In turn, the need for an organization dedicated to protecting student-athletes is more important than ever."

Referencing Northwestern scholarship football players' effort to unionize and a National Labor Relations Board regional director's determination that that athletes are employees who can unionize, the letter says "if the NCAA were accomplishing its mission of protecting student-athletes from exploitive practices those efforts would be unnecessary and likely unsuccessful."

Last week, a House committee held a hearing on the unionization effort.

The senators' letter asks Emmert to respond to a 10-point request list that includes:

Copies of "all current agreements to which the NCAA is a party regarding the transmission rights to NCAA content."

Copies of all current NCAA policies "related to the organization's use of student-athletes' likeness or identifying information/characteristics (e.g. jersey number or performance statistics) for profit."

A summary of each lawsuit or settlement between the NCAA and student-athletes since Jan. 1, 2012.